1. Field of the Invention
The present invention relates to an improvement of a focus or distance detecting apparatus mounted on single-lens reflex cameras.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. Hei-1-222583 discloses that the accumulating operation of an accumulation-type sensor used for detecting a focus is controlled such that the maximum and minimum values of an accumulation signal are detected while a photoelectric conversion cell performs an accumulating operation, and the accumulating operation is terminated when the difference between the maximum and minimum values reaches a predetermined level, in order to obtain a clear image signal even from a low-contrast object.
Cameras have been developed in each of which a passive-type focus detecting apparatus employing a sensor that operates in the above-discussed accumulation control method is used to perform focus detection at a plurality of focus detection areas and to automatically select the best focus point among the plurality of detected focuses to bring the camera into focus.
In the above-discussed conventional accumulation control method, when an object has a high luminance with a low contrast, if the maximum value of an accumulation signal reaches a predetermined level, even if the difference between the maximum and minimum values of the accumulation signal has not yet reached a predetermined value, the accumulating operation is terminated so that the accumulation signal of a photoelectric conversion cell is not saturated. When an object has a low luminance, since signal accumulation at a photoelectric conversion cell takes a long time and the dark current of the photoelectric conversion cell greatly affects, the accumulation time is limited to a predetermined time and the accumulating operation is terminated even if the difference between the maximum and minimum values of the accumulation signal has not yet reached a predetermined value.
To determine a focus detection area, from which information is obtained so that a camera is automatically brought into focus among a plurality of focus detection areas, focus detection is performed at all focus detection areas and a focus detection area is determined from various points such as the position of the focus and the characteristics of an image signal obtained when the focus is detected. Focus detection areas generating low-contrast image signals are not used because they do not satisfy the determination standard of the characteristics of image signals.
Since a focus detection area is determined from the contrast of image signals and other data after image signals are read from all of a plurality of focus detection areas, unnecessary reading is performed at focus detection areas having low contrasts and actually not used, including areas where a sufficient signal level is not obtained from an accumulation-type sensor in cases when an object has a high luminance with a low contrast and when an object has a low luminance with a low contrast.